Investment Securities - Cost and Amortized Cost Maturity Schedule (Details) $ in Millions	Dec. 31, 2021 USD ($)
Cost or Amortized Cost
Total fixed maturity investments	$ 1,895.5
Carrying Value
Total fixed maturity investments	1,908.9
Fixed Maturities and Convertible Fixed Maturities Excluding Mortgage Backed Securities
Cost or Amortized Cost
Due in one year or less	136.7
Due after one year through five years	866.2
Due after five years through ten years	365.7
Due after ten years	113.2
Carrying Value
Due in one year or less	137.3
Due after one year through five years	865.0
Due after five years through ten years	371.5
Due after ten years	122.5
Mortgage and asset-backed securities
Cost or Amortized Cost
Mortgage and asset-backed securities and collateralized loan obligations	413.7
Carrying Value
Mortgage and asset-backed securities and collateralized loan obligations	$ 412.6